IRF510
MOSFET
For a
5-15 watt transmitter, the IRF510 is a
pretty good choice.
Its Drain-source breakdown voltage is
100V which gives a reasonable amount of
headroom.
(on mod peaks the supply voltage will be
24V , assuming a 12V powersource.
The RF voltage on the FET will be
something more than 2 times that, say
peak voltage of 60-70 Volts.
The input capcitance of the IRF510 is
quite low (Ciss ~= 150PF) which makes it
easy(er) to drive than larger mosfets
with higher gate capacitance.
Why not try to get more out of the
IRF510? The problem is Rds - the on
Resistance,
which is 0.4 ohms for the IRF510. The
output impedance seen by the FET is
roughly V*V/2P
where V is DC supply voltage and P =
power out.
If P = 10 and V=12 then the output
impedance is about 12*12/2*10 = 7.2 Ohms
If P = 20 and V=12 then the output
impedance is about 12*12/2*20 = 3.6 Ohms
As the value of Rds gets larger relative
to the output impedance the efficciency
drops
and you stat producing more heat and less
RF.
At the small end of the scale, you could
try MTP3055Es. They are tested at 7 Mhz,
but they are gate capacitance is about 3
times that of the IRF510, and thier
breakdown
voltage is rated at 60 Watts. If you can
drive them you may be able to get more
out
than an IRF510, assuming you adjust the
output network ...